The present invention concerns a mobile data medium containing data in the form of marks on its surface, which are optically readable by laser beam and form a set of tracks, a process for handling an error signal indicating a tracking problem, and also the device for reading the mobile data medium.
The marks on the medium can have different forms. For example, they may be engraved or they may protrude. In the rest of this description, the marks are taken to be engraved and of oblong section.
The dimensions of these marks are chosen to be approximately equal to the wavelength of the laser beam. Therefore the interaction of the reading beam with a mark generates a diffracted wave; the dispersion of the radiation then indicates the information represented by the marks.
In the case of mobile media in the form of disks, which shall be taken as the example for the remainder of the description, the data is recorded along tracks comprising either a single spiral extending from the center to the edge of the disk, or a series of concentric circles. During reading the disk moves, rotating about an axis which is perpendicular to the surface of the disk and passes through its center. The tracking is therefore referred to as radial tracking.
It is known that the data stored on optical disks generally has one of two formats: composite continuous format and sampled format. "Format", refers to the arrangement of prerecorded information on the disk, used subsequently by the user to write and read data.
In the first case, the format information is distributed continuously along the whole length of the track, whereas in the second case it is in the form of successive blocks.
In the case of sampled format, each block comprises a header zone, known as a pre-format zone, followed by a zone reserved for the recording of user data. Each pre-format zone contains all the marks which, when read optically, facilitate the correct operation of the reading/writing device. There are four types of pre-format marks: focussing, radial tracking, access and synchronization.
The focussing marks, which can, moreover, be limited to a non-engraved zone, enable monitoring and correction of the focussing of the laser beam in the zone containing the tracks.
The radial tracking marks make it possible to correct any off-centering of the beam in the radial direction.
The access marks allow access to the track of a given address.
The synchronization marks make it possible to control the clocks ensuring the correct operation of the various reading/writing phases.
If no form of correction were provided, the radial tracking of the track, like the focussing or access, would constantly require almost perfect positioning of the reading head relative to the disk. This can not be achieved with reading devices and disks which are required to be easy to manufacture.
This leads to application of real-time control of the radial tracking. In the case of the sampled format, marks placed at regular intervals make it possible to correct any off-centering of the reading beam due to imperfect relative positions of the reading head and the disk. In the case of the composite continuous format, a continuous or almost continuous groove is used.
The means of radial tracking which then guarantee the processing of the data coming from the tracking marks include, amongst other things, optical sensing of the tracking deviation from the groove, and a device for displacing the reading beam, which is activated to compensate these deviations.
For example, the method known as "push-pull" can be described, which consists of using two photosensitive cells each sensing a fraction of the radial tracking beam.
The two cells are mounted on each side of the optical axis of the beam, and the electrical signals produced by these two cells are transmitted to the inputs of a differential amplifier. When the spot of the reading beam is centered on the track, the output signal of the amplifier is null. When the beam is off-center, the output signal of the amplifier represents the direction and amplitude of the off-centering.
The output signal of the differential amplifier is, in every case, transmitted to a radial position controller which corrects the off-centering.
If sampled format is adopted, the tracking device includes sampling means provided at the output of the photodetection means in order to sample the photodetected signal during previously chosen time intervals: those during which the reading spot crosses the radial tracking marks.
There are two known methods of improving the quality of this type of radial position control.
The first consists of applying to the laser beam a slight oscillation of constant frequency about the average axis of its path.
The second method consists of periodically placing one or several tracking marks slightly off-center in relation to the average axis of the laser beam path, which is not in this case subject to any oscillation. This second means produces the same effects as the first.
It is easy to understand the disadvantages associated with each of these methods described above:
In the first case an oscillation must be applied to the laser beam; in the second case, during manufacture of the disk, an optical beam deflector system is required to create the off-center marks.